Hi Michael,

I don't know if you're an Emacs guy but both org-mode and markdown-mode in Emacs have great outlining. Headings are collapsed/expanded using TAB key, there's easy navigation of large documents. I have a 26K word org file and a 38K word markdown file. Markdown-mode in Emacs probably wasn't meant for such a large file, somewhere between 30K and 35K markdown-mode started getting gippy. I don't know if org-mode would perform any better with such a large doc.

Take a look at org-mode http://orgmode.org/